Synthetic calcium carbonates are known chiefly for uses as fillers (paper, paint, plastics . . . ). For these applications it is particularly known to synthesize dense carbonates, calcite in particular having scalenohedral, rhombohedral or prismatic morphology or aragonite having acicular morphology.
Compositions containing synthetic calcium and magnesium carbonates are also known, used in particular in the field of paper manufacture and plastic materials.
For example compositions can be cited according to patent applications EP-A-2322581 and DE-A-3245103. The first discloses plate-like particles of magnesium carbonates in the form of hydromagnesite in a mixture with colloidal calcium carbonate, well-suited as fillers for paper or plastic materials. The second discloses scalenohedral particles of calcium carbonate (i.e. of rice grain shape) in a mixture with plate-like particles of basic magnesium carbonate. The respective carbonates do not form a composite.
The compositions described in European patent EP-B-0526121 are formed of a uniform mixture of particles of calcium carbonate having an acicular aragonite crystalline structure with columnar particle configuration and particles of slaked magnesium carbonate with plate-like particle configuration. These compositions are obtained by carbonatation of aqueous solutions of calcium and magnesium hydroxides using a complex process requiring the use of a variable CO2 flow rate.
A Japanese patent application JP-63-123810 is also known which describes a process for producing a mineral composition containing a mixed solid phase of calcium and magnesium carbonates. This process comprises carbonatation of an aqueous suspension of basic magnesium carbonate to which is continuously added an aqueous suspension of magnesium hydroxide and calcium hydroxide. Carbonatation is conducted with fast CO2 flow rate which gives rise to the formation of spherical, porous particles of basic magnesium carbonate on the seed nuclei of basic magnesium carbonate initially contained in suspension, the pores of these particles being partly filled with calcium carbonate. This composition is very dense and intended for the production of fillers for the manufacturing of paper, paints, plastic materials, or for the medical sector.